1. Field of the Invention
The invention pertains to the art of microwave radiometry using comparison of the amplitude of a signal to be investigated, such as a thermal noise signal, to the amplitude of a locally generated reference signal. The invention more particularly relates to a novel gain control reference generator system for use in an amplitude comparison radiometer receiver for the purpose of precision control of the over-all gain of the receiver, a reference generator operating by cyclic modulation of the temperature of a planar micro-resistor of unique design.
2. Description of the Prior Art
Prior art reference high-frequency noise sources include transmission line terminations and noise generating semiconductor and other diodes, including avalanche diodes, temperature limited thermionic vacuum diodes, and gas discharge devices. Such prior devices suffer various disadvantages, including large size and weight, high power consumption, and excessive cost. Other such devices demonstrate instability in time and temperature, general unsuitability for use at millimeter wave lengths, and lack of flexibility. Certain of the devices produce an output changeable in amplitude only slowly or by only a small percentage. The prior art devices, especially those imitating prior art planar infrared sources, have excessively high thermal capacity and are difficult to match efficiently in microwave circuits.
One of the relatively attractive noise reference sources of the prior art is that of the R. E. Wilt U.S. Pat. No. 3,693,095 for a "Radiometer Gain Control Reference", issued Sept. 19, 1972 and assigned to Sperry Rand Corporation. In the Wilt patent, the noise reference signal is generated by the square wave modulation of a fine heated wire whose electromagnetic noise energy spectrum is coupled into the path of the primary radiometer signals. Modulation is effected by causing a bridge circuit including the heated wire in one of its arms to become cyclically rebalanced in a square wave manner.
Wilt's heated wire device is smaller than other prior art devices except the semiconductor noise diode, but the output of the latter unpredictably varies with ambient temperature. Further, the heated wire device consumes less power than is consumed in the controlled heating of transmission line terminations. The heated wire device is useful at frequencies higher than the useful range of the vacuum diode. The gas discharge tube suffers in comparison because of its size, high voltage, and high current requirements.